Why? Which wire you can use in which situations is a first year apprentice question. Presumably the PV folks in your class are at least first year apprentices.
Or put another way, this is an ELECTRICIAN question, not a SOLAR INSTALLER question.
Actually, I am the only one in my class, instructor included, that has been through the apprenticeship. As such, I have caught and corrected many errors. The class I am in is the very first of it's kind for this college. I purposely sought to be in the first class as I knew I would get to be part of it's evolution. The instructor and I spend a great deal of time both in person and via e-mail about the interpretation and proper application of the NEC.
Our text if full of omissions and ripe with errors. One illustration is of a gas turbine power plant and it has the generator in the exhaust of the jet engine. I know from actually being part of the construction of a jet turbine plant that the generator is on the intake side of the engine and takes advantage of the intake air to cool the generator. (Duh...) Otherwise, it would have to be cooled using liquid hydrogen.
The text never mentions the small conductor limitation and lists #10 as good for 40 amps. They also list ampacities for 16 and 18 AWG and fail to mention 310.5's restriction to a #14 minimum, or 110.14's temperature rating limitation. Due to the omission of 110.14, ALL the example calculations are done using 90C as all the conductors in the examples are 90C rated.
Our test tells us that the higher the temperature of a battery, the better as they use T vs. output curves and do not so much as mention the ill effects of high temperature on storage batteries. I forwarded some charts that include the life expectancy deterioration curves along with output. At 140F or so, the output increases 20 percent but the life decreases by 80 percent. The optimal temp for a storage battery is 77F, not as hot as we can keep them. That info was added to the power point stuff.
The instructor is also changing the power point examples to account for 110.14. 110.14 is a nasty little critter. Read the entire thing and tell me if your head doesn't spin. Not only is (C)(1) cryptic as it discusses 'termination provisions' but does not define them, if we call a breaker a device we now have a very low temp to compensate for. In the case of a Homeline 20A, it's terminal is rated for 60 or 75C, but 40C is molded into the case. Does that mean we have to derate down to 40C?
The instructor is a great guy and is very appreciative of my help and interest. I had him and his family out to a large amateur radio event this weekend. They got to chow down and get a tour of our emergency communication capabilities. He is an electrical engineer and actually builds stuff with his own hands. A rare bird indeed. Some of our instruction is based upon installations he did himself. We planned on bringing three of the school's 225 watt panels out to the event (we would have got extra points for using solar power), but at the last minute the college bean counter said no. Once the instructor saw that the event was being coordinated by a cop and we had the county's quarter million dollar mobile command center there, he told me that next year we would have our panels. I offered to have them picked up in a police cruiser, if needed.
So, if you were under the impression that the students and instructor have at least a first year level of NEC instruction, you would be incorrect.
Thanks for sharing your experiences, BTW. Even though we may not agree on everything (like negative PF) you bring to the table real world scenarios that most of us would never have insight to.
